The many faces of synapsid cranial allometry

Isaac W Krone,Kenneth D Angielczyk,Christian F Kammerer

doi:10.1017/pab.2019.26

Abstract

AbstractPrevious studies of cranial shape have established a consistent interspecific allometric pattern relating the relative lengths of the face and braincase regions of the skull within multiple families of mammals. In this interspecific allometry, the facial region of the skull is proportionally longer than the braincase in larger species. The regularity and broad taxonomic occurrence of this allometric pattern suggests that it may have an origin near the base of crown Mammalia, or even deeper in the synapsid or amniote forerunners of mammals. To investigate the possible origins of this allometric pattern, we used geometric morphometric techniques to analyze cranial shape in 194 species of nonmammalian synapsids, which constitute a set of successive outgroups to Mammalia. We recovered a much greater diversity of allometric patterns within nonmammalian synapsids than has been observed in mammals, including several instances similar to the mammalian pattern. However, we found no evidence of the mammalian pattern within Therocephalia and nonmammalian Cynodontia, the synapsids most closely related to mammals. This suggests that the mammalian allometric pattern arose somewhere within Mammaliaformes, rather than within nonmammalian synapsids. Further investigation using an ontogenetic series of the anomodontDiictodon felicepsshows that the pattern of interspecific allometry within anomodonts parallels the ontogenetic trajectory ofDiictodon.This indicates that in at least some synapsids, allometric patterns associated with ontogeny may provide a “path of least resistance” for interspecific variation, a mechanism that we suggest produces the interspecific allometric pattern observed in mammals.

Highlights

The cranium is a highly complex structure that has evolved myriad forms associated with many different ecologies throughout the tetrapod radiation
Analyses of Allometry All Nonmammalian Synapsids.—There is a significant but weak relationship between size and shape in our data set of all synapsid crania (R2 = 0.0197, p < 0.001) (Table 2), but individual synapsid groups do not share allometric trajectories
The central prediction of CREA is that larger animals will have proportionally longer snouts, a pattern based on relative lengths that could be recovered using three-dimensional, twodimensional, or simple linear morphometrics

Summary

Introduction

The cranium is a highly complex structure that has evolved myriad forms associated with many different ecologies throughout the tetrapod radiation. Despite this fact, tetrapod crania all exhibit a shared basic architecture— a braincase and associated sensory capsules and a facial skeleton associated with the jaws (Emerson and Bramble 1993)—facilitating comparisons across the group. Many tetrapod clades have an extensive fossil record including well-preserved crania. These clades provide excellent systems in which to study both small- and large-scale patterns in shape evolution over time. Little work has been done to synthesize the results of these analyses or to look for pan-tetrapod morphological patterns

Methods

Results

Conclusion